The present invention relates generally to electrical power supply circuits and in particular to circuits for controlling inrush current when power is initially applied to filter capacitors such as those preceding a DC-to-DC converter.
Power supply circuits, such as those for converting between direct current (DC) voltages or between alternating current and direct current of the same or different voltages, may employ one or more filter capacitors used to accommodate varying loads of a switching circuit or power provided by an AC waveform.
Referring to FIG. 1, a typical prior art power supply circuit 10 may include a power source 12, for example, a twelve-volt battery, providing current through a fuse 14 and control switch 16 to a filter capacitor 18.
The filter capacitor 18 may span the input terminals of a load, such as DC-to-DC converter 20, as is well known in the art. The filter capacitor 18 serves to store power to accommodate the fluctuating demands of the DC-to-DC converter 20 and its load 22.
When the power source 12 is initially connected, by closing of switch 16, to an uncharged filter capacitor 18, high inrush currents will pass from the power source 12 to the filter capacitor 18. Typical inrush currents can be ten times the rated current of the DC-to-DC converter 20. These high inrush currents require that the switch 16 be of high current carrying capacity and may require increasing the size of the fuse 14 to a value higher than that which would be preferred for protection of other circuit elements. The high inrush current may also create an arc across the contacts of the switch 16 which can require that the switch be a sealed switch if the environment in which the power supply is being used contains combustible fumes.
Referring still to FIG. 1, high inrush currents may be moderated by placing a negative temperature coefficient (NTC) resistor 24 in series with the current flow to the filter capacitors 18. Such an NTC resistor 24 is initially cool and has a high resistance value limiting inrush current when the power source 12 is first connected. After a period of operation, current flow through the NTC resistor 24 warms it, lowering its resistance.
Variation in the resistance of the NTC resistor 24 combined with variation in the voltage of the power source 12, for example, a lead acid battery, make the maximum inrush current difficult to characterize. If the switch 16 is cycled off and then on again, a high inrush current will occur if the capacitor 18 has discharged but the NTC resistor 24 has not cooled. The NTC resistor 24 continually dissipates power, reducing the efficiency of the power supply circuit 10 and reducing the charging rate of the capacitor 18 more than necessary as a result of inevitable mismatch between heating and charging rates.
Referring to FIG. 2, the problems of power dissipation and unpredictable maximum inrush current can be reduced by using a fixed resistor 26 to limit inrush current and shunting this fixed resistor 26 with a transistor 28 after a time delay, effectively removing the resistor. A timer 32 is triggered by a signal from the DC-to-DC converter 20 or switch 16 and turns on transistor 28 shorting resistance 26 after a time delay during which it may be assumed that filter capacitor 18 has been fully charged. The transistor 28 is operated in fully “on” or “off” states so as to minimize its power dissipation
This approach still presents the risk that a cycling of switch 16 could create high inrush currents. And, although the fixed resistor reduces variation in maximum inrush current caused by the variability of the NTC resistor 24, maximum inrush currents will still vary as a function of the voltage of the source 12. The time delay of timer 32 must be set to a compromise value that inevitably reduces the charging speed of the capacitors more than necessary.